The present invention relates to controls for heating, ventilating, and air-conditioning (HVAC) systems, specifically to controlling a fan speed control for HVAC systems and calibrating HVAC systems.
Modern buildings can have complex HVAC systems to control indoor temperature, pressure, ventilation rate, and other variables in a way that makes efficient use of energy. One way to conserve energy in these systems is to use a so-called variable-air-volume design. Key components of a variable-air-volume system are a fan, a fan motor, and a fan speed modulation device. The fan is a prime mover that causes air to move. The motor, which is typically an alternating current motor for HVAC fans, converts electrical energy to mechanical energy to operate the fan. A fan speed modulation device is typically a variable frequency drive (VFD) for an alternating current motor.
Some VAV systems were originally designed as constant volume HVAC systems, then converted later to VAV operation to conserve energy. Some of these conversions leave the original terminals in place. Constant volume terminals either do not have dampers or they have mixing dampers that mix hot air with cold air. Constant volume terminals without dampers are called reheat terminals. Constant volume terminals with mixing dampers are called dual-duct terminals. Sometimes the dual-duct terminals are located at the air-handling unit, in which case the air-handling unit is usually referred to as a multi-zone system.
One control strategy for the fan of variable-air-volume systems is to regulate a static pressure in a duct at a point downstream of the fan. In large systems or dual-duct systems, it is common to measure the duct pressure at more than one point and control the minimum reading to a setpoint. This strategy seeks to keep the static pressure at a measurement point constant at all times. Control strategies based on a constant static pressure in the duct have been proposed in U.S. Pat. No. 4,437,608 to Smith (1984) and U.S. Pat. No. 6,227,961 to Moore et al. (2001). U.S. Pat. No. 4,836,095 to Wright (1989) describes a variant of this strategy for systems that have multi-speed fans rather than fans in which the speed is continuously variable. A rule of thumb for this strategy is to locate the pressure sensor two-thirds of the distance from the fan to the end of the duct. A problem with this strategy is that it is inefficient at part-load conditions, when the flow rate is significantly lower than a design flow rate, which is the flow rate at which the system should operate when the fan is running at full speed.
Another control strategy that overcomes the problem of constant static pressure control is one in which a static pressure setpoint is reset based on a position of a terminal damper that is most open. Control strategies that reset the static pressure based on the position of the terminal damper that is most open have been proposed in U.S. Pat. No. 4,630,670 to Wellman and Clark (1986) and U.S. Pat. No. 5,863,246 to Bujak (1999). An objective is to keep this damper nearly open or completely open. Doing so reduces throttling losses at part-load conditions. One problem with resetting static pressure based on the position of the most-open terminal damper is that it requires that the control system be able to measure the position of every terminal damper. Large systems could have hundreds of terminal dampers. Requiring terminal damper position measurement adds cost to the HVAC system.
Another problem with resetting static pressure based on the position of the most-open terminal damper is that it is sensitive to a communications failure. The terminal dampers are usually located far from the fan, so a digital communication network is used to connect the terminal unit control device, which knows the terminal damper position, with the fan control device. A failure in the network connecting these devices will cause the control strategy to fail.
Yet another problem with resetting static pressure based on the position of the most-open terminal damper is that it is sensitive to a terminal unit failure. If one of the terminal units is not working properly, then the resetting strategy will not work properly.
Yet another problem with resetting static pressure based on the position of the most-open terminal damper is that it is sensitive to a design flaw in which one or more terminal dampers is undersized. In this case the undersized terminal damper will require high pressure to achieve its required flow, causing large throttling losses at the terminal dampers that are not undersized.
Yet another problem with resetting static pressure based on the position of the most-open terminal damper is that it is difficult to tune. The most efficient operating point is when the most-open damper is completely open. If the strategy tries to keep the most-open damper completely open then the strategy cannot determine if the duct pressure is too low. If the controller tries to keep the most-open damper nearly completely open, then when it becomes completely open due to a disturbance in the system, the strategy cannot determine if the pressure is just slightly too low or far too low.
Several variants of static pressure resetting have been used. For example, one strategy resets the static pressure based on an average position of a set of terminal dampers that are most open. The averaging feature allows this strategy to reduce the energy consumption at part load conditions even if a small number of terminal units fail or are undersized. However, the strategy is still limited by the need for terminal damper position sensing, is still sensitive to network failure, and is still difficult to tune.
A strategy for modulating the fan of HVAC systems originally designed for constant volume operation is described in United States patent application 20060161306 to Federspiel. This strategy uses discharge air temperature sensors as feedback for adjusting the fan speed. That strategy attempts to keep the highest discharge air temperature as high as possible or the lowest discharge air temperature as low as possible so that the zones are heated or cooled with a higher absolute temperature difference between the discharge and the zone, but at reduced flow.
More complex strategies for controlling fans have been proposed in U.S. Pat. Nos. 5,540,619 and 5,573,181, both to Ahmed (1996). These inventions require the measurement of flow or pressure in all branches downstream of the fan in addition to measurement of the position of each terminal damper. Consequently, they have all the problems of the static pressure resetting inventions described above.
Accordingly, a need exists for a fan control strategy that can improve the part-load efficiency of fans in variable-air-volume systems without requiring the added cost of position measurements, without being sensitive to communications system failure, and being easy to calibrate.